1. Field of the Invention
The present invention relates to a method for image compression coding. More particularly, the present invention relates to a coding method for compressing images through trellis coded quantization (TCQ).
2. Description of Related Art
Trellis coded quantization is derived from trellis coded modulation (TCM). In recent years, trellis coded quantization is applied to various coding techniques for it has great compression capability. For example, trellis coded quantization is used as another option of scalar quantization in image compression standard JEPG2000 for improving image compression capability. In addition, trellis coded quantization is used as the coding technique in both the patent titled as “image encoder method” disclosed in the U.S. Pat. No. 6,535,647 and the patent titled as “digital signal coding and decoding based on subbands” disclosed in the U.S. Pat. No. 6,501,860.
With the great image compression capability of trellis coded quantization, Marcellin and Fischer also published a study about “trellis coded quantization of memoryless and Gauss-Markov sources” in “Transactions on communications”, pp. 82˜93, issued by IEEE (Institute of Electrical and Electronic Engineers) on 1990. According to this conventional technique, a trellis diagram illustrated in FIG. 1 is used for coding, wherein each node in FIG. 1 denotes one of states S0˜S3, and a/b on each branch path denotes the input code/quantized subset from the previous state to the current state of the branch path.
During the coding procedure, a codebook is first looked up to obtain the quantization value of a quantized subset on each branch path in the trellis diagram. After that, the total error of each path is calculated according to the input message sources and the quantization value of the quantized subset on each branch path. A minimum error path is then selected from a plurality of paths as the coding path of the message sources, as the solid line in FIG. 1. Finally, a quantized codeword corresponding to each pixel is located by looking up the same codebook to obtain the encoded codeword.
As described above, the conventional trellis coded quantization is only suitable for coding in a frequency domain because the conventional trellis coded quantization is only suitable for frequency domain signals of low correlation since only one codebook is used for coding. Contrarily, if the conventional trellis coded quantization is used for spatial domain coding, coding distortion will be increased and further the encoded image quality will be affected. In other words, the image compression quality will be reduced if the conventional trellis coded quantization is used for spatial domain coding.